In a mobile communication system which is widely used at present, a service area as a communicable area for providing a service is divided into relatively small wireless zones that are called cells. In such a system, a plurality of base stations are arranged to cover the wireless zones, and a mobile station communicates with the plurality of the base stations by setting wireless frequency channels. If a same frequency channel is assigned to adjacent cells, interference between the cells is maximized in an end portion of the cell, so that the maximum throughput is greatly reduced, as compared with the center of the cell. Therefore, a conventional technique is known in which different frequency channels are used in the adjacent cells in order to prevent interference between the adjacent cells, as described in, for example, Japanese Laid Open Patent Application (JP-A-Heisei 10-93529: first conventional example).
FIG. 1 shows frequency channels to be assigned in the first conventional technique. As shown in FIG. 1, a base station is arranged in each of cells, and different frequency channels f1, f2 and f3 are assigned to the cells A, B and C respectively. Thus, by assigning the three frequency channels, or by repeatedly assigning different frequency channels over a frame, the distance between the cells causing interference from each other is elongated. Consequently, the maximum throughput can be improved in an end portion of the cell, compared with a case of using a single frequency channel.
However, if the different frequency channels are assigned to adjacent cells, a frequency utilization efficiency is greatly decreased in a center portion of the cell or around the base station, compared with a case of assigning the same frequency channel to the adjacent cells.
In conjunction with the above description, a wireless access system is disclosed in Japanese Laid Open Patent Application (JP-P2001-157249A). In the wireless access system of this conventional example, a service area as a whole is divided into cells, and a base station and a plurality of subscriber wireless stations for performing a point to multipoint communication by using the base station through wireless channels are arranged in each of the cells. An uplink wireless channel in a direction from the subscriber wireless station to the base station is composed of a wireless channel having a small capacity with more frequency repetition. A downlink wireless channel in a direction from the base station to the subscriber wireless station is composed of wireless channels having a larger capacity with less frequency repetition than the uplink wireless channel. Thus, a repetition frequency is asymmetrical between the uplink wireless channel and the downlink wireless channel. Therefore, while maintaining C/I of the uplink and downlink channels to be substantially equal, adaptability to a communication service in which the downlink channel requires a larger transmission capacity than the uplink channel can be maintained.
Also, there is a CDMA mobile communication system disclosed in Japanese Laid Open Patent Application (JP-P2001-119743A). This CDMA mobile communication system as this conventional example includes a plurality of base stations and mobile stations. A repetition period is equal to an information symbol period, and a short period spreading code commonly used in each of the base stations and the repetition period are longer than an information symbol, and the information symbol is spread in duplicate by using the short period spreading code and a long period spreading code which is different in each of the base stations. Synchronizing signals which have spread by the plurality of the base stations by using only the short period spreading codes are continuously transmitted. The synchronizing signal is transmitted from the plurality of the base stations at transmission timing which is shifted by a predetermined period of time for each base station. Therefore, loads of searching a peripheral cell and selecting the base station in the mobile station is decreased.
There is also a communication system disclosed in Japanese Laid Open Patent Application (JP-P2003-18077A). In this conventional example, a single zone is composed of a plurality of cells formed by a plurality of base stations, in which a zone control station connected to the plurality of the base stations controls communication with a mobile station in the zone. The base stations positioned first and last in the zone are base stations to communicate with the mobile station in two ways, and the remaining base stations in the zone are base stations to communicate with the mobile station in a one way. Therefore, improvement of a throughput is intended in case of performing a communication between the mobile station and the base station in a communication zone in which the cells are scattered.
Furthermore, there is a mobile communication system disclosed in Japanese Laid Open Patent Application (JP-P2004-15697A). According to this conventional example, the mobile communication system of a CDMA system or a W-CDMA system includes a mobile station, a base station having the plurality of carrier frequency large areas, and a wireless network control apparatus. The wireless network control apparatus includes a state measuring section, a call control processing section, and a frequency load distributing unit. The frequency load distributing unit includes a frequency converting unit for updating state data for each frequency and next assignment control data that were prepared in advance by referring to a frequency state change notice transmitted from the state measuring unit, and a frequency assignment control unit for returning a frequency assignment response by selecting a frequency referring to the next assignment control data updated in response to a frequency assignment request containing current processing information to be transmitted from the call control processing unit.